- Email: [email protected]
Wear behavior of coated tools in laser assisted micro-milling of hardened steel
Wear 296 (2012) 510–518
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Wear behavior of coated tools in laser assisted micro-milling of hardened steel Mukund Kumar a, Shreyes N. Melkote n,a, Rachid M’Saoubi b a b
George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA R&D Materials and Technology Development, SECO Tools AB, Fagersta, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2012 Received in revised form 8 August 2012 Accepted 9 August 2012 Available online 30 August 2012
This paper presents a comparison of the performance of different cutting tool coatings for Laser Assisted Micro-Milling (LAMM). A thermal model is used to predict the temperature rise in the material removal surface which helps in analyzing the severity of the thermal conditions experienced by the cutting tool in LAMM. Machining experiments are then carried out to evaluate the wear behavior of different commercially available (TiCN, TiAlN) and customized coated tools (TiSiN, Al2O3, Al2O3 þ ZrN). These coatings were selected since they have the capability to withstand the temperatures experienced in LAMM. The results of micro-milling experiments indicate that commercially available coatings like TiCN perform poorly due to their inferior adhesion characteristics with the base material. Delamination is found to be the principal wear mechanism of TiCN, TiSiN, and Alumina (Al2O3) coated tools for the conditions investigated in this study. In addition, the results indicate that the wear performance of TiAlN and Al2O3 þ ZrN coated tools is superior. & 2012 Elsevier B.V. All rights reserved.
Keywords: Tool wear Coatings Laser assisted micro-milling
1. Introduction Micro-milling is increasingly being used to fabricate micromolds for injection molding [1,2]. Hardened steels and ceramics are the preferred materials for micro-molds because of their ability to withstand high cyclic thermal and mechanical loads [3]. In order to create features in these hard-to-machine materials, new hybrid processes have been proposed [4,5]. Laser Assisted Mechanical Micro-milling (LAMM) is one such process [6]. Recent work on LAMM has demonstrated its potential for micromachining hard materials [6]. However, a key limitation of the process today is the lack of affordable cutting tool materials that can withstand the high temperatures experienced in LAMM and also possess reasonable tool life. Conventional micro-milling tools (o500 mm dia.) are made of micro-grain tungsten carbide (WC–Co), whose strength starts to decrease at temperatures above 500 1C. In addition, tool wear becomes significant at high temperatures. A cost effective solution to this problem is the use of temperature-resistant coatings. The suitability of tool coatings employed in conventional macro- and micro-milling processes [7,8] for use in LAMM needs to evaluated since the heat generated in LAMM is significantly higher than in conventional processes. No work has been reported on the wear behavior of coated tools in LAMM.
n
Corresponding author. Fax: þ1 404 894 9342. E-mail address: [email protected] (S.N. Melkote).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.08.011
Consequently, the wear performance of carbide micro-end mills with commercially available coatings (TiCN and TiAlN) and custom coatings (Al2O3, Al2O3 þZrN) is studied through micro-milling experiments carried out on hardened A2 tool steel (62 HRC). The worn tools are characterized using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) to understand the dominant wear mechanisms of the tool coatings.
2. Thermal modeling Prior to carrying out the tests, the temperature distribution in the workpiece surface directly in contact with the tool is analyzed via an experimentally calibrated moving heat source model to estimate the thermal loads experienced by the tool in LAMM and to aid in the selection of suitable cutting tool coatings. A standard moving heat source model is used to calculate the temperature distribution in the workpiece surface in contact with the tool (termed the material removal surface in this paper). Specifically, the temperature distribution is determined using an analytical model of a moving point heat source [9] over a semi-infinite medium as given by Eq. (1) Z Z 0 Tðx,y,zÞ ¼ a qððx0 ,y0 Þ=2pKsÞeðU=2kÞðsðxx ÞÞ dx0 dy0 , qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1Þ s ¼ ðxx0 Þ2 þ ðyy0 Þ2 þ ðzÞ2 : where, a is the absorptivity, q is the heat intensity (W/m2), K is the thermal conductivity (W/m-K), k is the thermal diffusivity (m2/s), and
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U is the velocity of the heat source (m/s). The thermal conductivity of the work material is modeled as a temperature dependent function given by Eq. (2) [10]. K ¼ 18:7 þ 0:0138 T
ð2Þ
The temperature dependence of thermal conductivity requires an iterative method to solve Eq. (1). The laser beam is modeled as a 280 mm diameter Gaussian heat source and the laser power is fixed at 18 W. The distance between the trailing edge of the laser spot and the leading edge of the cutting tool is fixed at 60 mm. The temperature distribution below the workpiece surface calculated from the model assuming unit absorptivity is shown in Fig. 1. The scanned specimen is sectioned, polished and etched to reveal the microstructure shown in Fig. 2. It clearly shows the demarcation between the phase transformed or laser hardened region (white color) and the tempered region (black color) created by the laser scan. The depth of the phase transformed region, d, is measured using an optical microscope. The temperature at this depth is taken to be the Ac3 temperature of A2 tool steel, which is 793 1C. The absorptivity coefficient in the thermal model is calibrated by dividing the temperature at depth d computed from the thermal model by the nominal Ac3 temperature of A2 tool steel. This calibration approach has been used successfully by other researchers [11,12]. Using this approach, the absorptivity for the conditions given in Fig. 1 is found to be 0.793. Note that it is possible to predict the depth of the phase transformed region without measuring it each time, provided the temperature dependence of thermal conductivity and specific heat, and the dependence of surface roughness on the absorptivity of the laser beam are known precisely [10]. However, in practice it is often difficult to obtain these values for numerical computations. Hence, the above model calibration approach is used in this study [9]. Once the calibrated absorptivity is known, the thermal model can be used to determine the temperature variation in the curved
Fig. 2. Micrograph of laser scanned surface showing the phase transformed region (white color) for the conditions given in Fig. 1 (d is the depth of phase transformed region).
Fig. 3. (a) Position of tool with respect to the laser beam (A–B indicates the line along which the temperature is predicted), (b) Predicted temperature variation in the workpiece along A–B and E–F (laser power: 18 W, spot size: 280 mm, scan speed: 100 mm/min, laser-tool distance: 100 mm).
Fig. 1. Temperature distribution (in 1C) in the X–Z plane (shown in Fig. 3) due to a laser scan along the indicated direction (laser power: 18 W, spot size: 280 mm, scan speed: 100 mm/min, laser-tool distance: 100 mm, absorptivity: 1).
material removal surface at the leading edge of the tool. The relative positions of the laser beam and the tool used in the LAMM experiments are shown in Fig. 3(a). Under these conditions, the temperature distribution is determined at the front edge of the tool along the lines A–B and E–F in Fig. 3(a). The temperature rise is found to vary between 300 and 450 1C on the curved material removal surface, as seen in Fig. 3(b). However, this temperature rise is only due to laser heating. The heat
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Table 1 Experimental conditions. Cutting tool Spindle speed Feed rate Depth of cut Workpiece material Laser power and spot size Distance between the center of the laser spot and the leading edge of the tool
200 mm dia. WC–Co, 2 flute end mills, and 250 mm dia. WC–Co 2 and 4 flute end mills 50,000 rpm 200 mm dia. tools—220 mm/min 250 mm dia. tools—100 mm/min 25 mm A2 Tool Steel (62 HRc) 18 W, 280 mm 200 mm
generated due to cutting is not considered in this study. Thus, the temperature rise experienced by the tool will be higher than shown in Fig. 3. It is evident from Fig. 3(b) that the temperature rise of 300–450 1C in LAMM is significant when compared to the estimated temperature rise of only 100–200 1C in micro-milling of 1018 steel and Al6061-T6 without laser heating [13]. Consequently, the coated tools must be capable of withstanding such high temperatures in the LAMM process for them to be viable.
Table 2 Cutting tool coatings. Commercially available Custom coated
TiAlN, TiCN TiSiN, Al2O3, Al2O3 þ ZrN
3. Micro-milling experiments with coated tools Laser assisted micro-milling experiments were conducted on A2 tool steel (62 HRc) using the experimental conditions given in Table 1. The tools used were 200 and 250 mm diameter end mills obtained from different tool manufacturers. The slotting experiments were conducted at manufacturer recommended feed rates and depths of cut. A few tests were conducted without laser assist to compare with the laser assist cases. All experiments were performed dry. The tool coatings evaluated in this study are listed in Table 2. These coatings were selected since they have the capability to withstand the high temperatures experienced by the tool in the LAMM process. Note that TiAlN and TiCN coatings are available from select cutting tool manufacturers. Tools were also custom coated with TiSiN, amorphous alumina (Al2O3), and Al2O3 þZrN in collaboration with a cutting tool manufacturer and a coatings supplier, respectively. All coatings were deposited on the cutting tools using the Physical Vapor Deposition (PVD) process. TiAlN and Al2O3 were PVD sputter coated, and TiSiN was coated using the arc evaporation technique. The TiSiN coating has been recently shown to outperform TiAlN and TiN coatings in terms of better abrasive wear resistance and fatigue behavior in conventional milling experiments [14]. The Al2O3 coating was selected because of its higher thermal stability and chemical inertness [15]. The thickness of TiSiN and Al2O3 coatings ranged from 2.5 to 3 mm. The thickness of the ZrN interlayer, which is used to promote adhesion of the alumina coating to the carbide substrate, is around 0.5 mm. The worn tools were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) to understand the dominant wear mechanisms.
4. Wear behavior of coatings 4.1. Uncoated tool Fig. 4(a) shows an SEM micrograph of the uncoated tungsten carbide micro-end mill after a 25 mm length of cut. The micrograph reveals extensive rubbing on the flank face of the uncoated tool (indicated by the arrows). Edge rounding without edge chipping is also observed, which indicates gradual abrasive wear of the tool. In addition, as seen in Fig. 4(a), adhesion of the softened work material to the cutting tool surfaces is also observed.
Fig. 4. (a) SEM micrograph of uncoated tool after 25 mm of micro-milling, (b) EDS spectra of the uncoated tool (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm) from the region indicated in Fig. 4(a). The EDS spectra obtained from this region confirms the presence of oxygen (O).
Visual examination of the used tool indicates some discoloration, which is attributed to oxidation of the uncoated carbide tool. Tungsten carbide begins to oxidize at temperatures above 500 1C [16]. As predicted by the thermal model, the workpiece is heated
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Fig. 5. (a) SEM micrograph of a new TiCN coated tool, (b) SEM micrograph of a TiCN coated tool after a 25 mm length of cut, (c) EDS Spectra of TiCN coated tool (from the top left region near the cutting edge indicated in Fig. 5(b)), (d) EDS Spectra of TiCN coated tool (from the flank face as indicated in Fig. 5(b)) (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
Fig. 6. (a) SEM micrograph of the TiAlN (SECOs Mega-T) coated tool after a 25 mm length of cut with laser assist, (b) EDS Spectra of TiAlN coated tool (from around the cutting edge), (c) End cutting edges indicating minimal wear (feed rate: 220 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
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to a temperature of 450 1C. In addition, there is temperature rise due to shear and frictional heating produced by the cutting process. Hence, the total temperature rise is expected to be closer to 500–600 1C in LAMM. The ability of the tool to resist wear at these high temperatures will directly determine tool life.
Fig. 5(d) indicates the overwhelming presence of titanium (Ti), carbon (C) and nitrogen (N), which confirm the composition of the coating material. Based on these findings, delamination wear is the dominant mechanism that governs the wear behavior of TiCN coated micro-tools in LAMM.
4.2. TiCN coated tool
4.3. TiAlN coated tools
SEM micrographs of the TiCN coated tool before and after a 25 mm length of cut with laser assist are shown in Fig. 5(a) and (b), respectively. Fig. 5(b) shows clear evidence of coating delamination near the cutting tool edge. The coating has been completely removed but the cutting edge is still intact. In addition, there is rounding of the cutting edges indicating gradual tool wear. Improvement in wear performance will be obtained if the surface roughness and adhesion of the coating to the WC–Co substrate can be enhanced. Delamination of the coating is confirmed by the EDS spectra (Fig. 5(c) and (d)) corresponding to the two regions marked in Fig. 5(b). The spectra shown in Fig. 5(c) reveals the presence of only tungsten (W) and carbon (C), which indicate that the TiCN coating has been removed from these regions. These observations are in line with the results of Aramcharoen et al. [7], who observed that the TiCN coating delaminates because of poor adhesion to the base substrate when micromilling hardened tool steel (45 HRc) with a 500 mm micro-milling tool, albeit without laser assist. This is also confirmed by their pin-on-disc experimental results [7], which show that the TiCN coating has a higher hardness (3178 HV) and a lower adhesive failure load of 48 N compared to the TiN coating, which has a hardness of 2738 HV and a adhesive failure load of 60 N.
A SEM micrograph of the TiAlN coated tool (SECOs Mega-T) after 25.4 mm of cutting with laser assist is shown in Fig. 6(a). The principal wear mechanism of this coating is gradual abrasive wear, which is confirmed by the edge rounding observed in the SEM image. The EDS spectra from the cutting edge (shown in Fig. 6(b)) shows the presence of TiAlN. No delamination of the coating is observed, which indicates good adhesion of the coating to the substrate. In addition, significant adhesion of the softened work material to the cutting tool surfaces is observed and is confirmed by the presence of Iron (Fe) and (O) peaks in the EDS spectra. Fig. 7(a–c) show SEM micrographs of the TiAlN coated tool after a 25.4 mm length of cut without laser assist. It is evident that the coating is unable to withstand the tool stresses and therefore delaminates, which suggests that tool wear is higher without laser assist. At lower cutting temperatures (without laser assist), aluminum oxide is unlikely to form on the TiAlN coating, resulting in increased tool wear [7]. In contrast, as seen in Fig. 6(a), the coating is able to withstand the higher temperatures and lower interfacial stresses experienced by the cutting tool in LAMM. This might be due to the formation of an Al2O3 film on the TiAlN coating, which yields enhanced wear performance.
Fig. 7. SEM micrograph of the TiAlN (Mega-T) coated tool after a 25 mm length of cut without laser assist. (a) Delamination of coating, (b) Magnified SEM micrograph of (a), (c) Chipping of cutting edge, (d) EDS Spectra of TiAlN coated tool (from around the cutting edge) (feed rate: 220 mm/min, depth of cut: 25 mm).
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Fig. 8. (a) SEM micrograph of a TiSiN coated tool after a 25 mm length of cut with laser assist, (b) Magnified SEM micrograph of (a), (c) EDS Spectra of TiSiN coated tool (from the region near the cutting edge) (feed rate: 220 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
4.4. TiSiN coated tool Fig. 8(a) and (b) show SEM micrographs of the TiSiN coated tool after 25.4 mm of machining with laser assist. The coating is found to delaminate under the influence of the machining stresses. Additionally, the EDS spectra obtained from the region near the cutting edge is shown in Fig. 8(c) and confirms that the coating has peeled off. Therefore, delamination is the principal wear mechanism of this coating. Adhesion of the TiSiN coating with the WC–Co substrate has to be further improved for the coating to perform better. This has also been observed in conventional milling where the TiSiN coatings exhibited poor adhesion [14]. Fig. 9(a) and (b) show SEM micrographs of the TiSiN coated tool after 25.4 mm of machining without laser assist. The coating near the cutting edge has peeled off, which is confirmed by the EDS spectra shown in Fig. 9(c). Extensive adhesion of the work material with the cutting tool is observed, which can also cause delamination of the coating. Based on these results, it can be concluded that delamination is the dominant wear mechanism of the TiSiN coating in micro-milling with and without laser assist.
this study. Fig. 11(a) and (b) show SEM micrographs of the worn tool after micro-milling a 25 mm long slot with laser assist. The micrographs clearly indicate peeling of the Al2O3 coating. This is thought to be due to a combination of excessive rubbing at the micro-scale combined with poor adhesion of the coating to the tungsten carbide substrate. Kumar et al. [17] have used indentation to study the adhesion of Al2O3 coatings to a carbide substrate and report that the adhesion strength of a single layer Al2O3 coating to the carbide substrate is HF6 on a scale of HF1 to HF6, where 1 indicate the strongest adhesion. The EDS spectra shown in Fig. 11(b) from the region marked ‘A’ in Fig. 11(a) clearly indicates complete loss of the alumina coating. The EDS spectra from region ‘B’, shown in Fig. 11(d), shows the presence of constitutive elements (Al and O) of the coating. Note that the EDS spectra are obtained from a maximum substrate depth of 2 mm. The absence of tungsten and carbon in Fig. 11(d) confirms that the remaining thickness of the coating in this region is more than 2 mm. The cutting edge is still intact after 25 mm of cutting under the machining conditions listed in Table 1. Delamination due to inadequate adhesion strength of the coating appears to be the principal wear mechanism of the alumina coated tools in LAMM.
4.5. Alumina (Al2O3) coated tools 4.6. Alumina (Al2O3) þZrN coated tool Alumina (Al2O3) coating of thickness 2.5–3 mm was deposited on a two flute WC–Co micro-end mill using the PVD process. The coated tool before machining is shown in Fig. 10. The coating appears to be relatively smooth compared to the other coatings in
To improve adhesion of the alumina coating to the carbide substrate, a thin layer of ZrN (600 nm) was deposited on the tool before depositing the alumina layer. Note that ZrN coating has
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Fig. 9. (a) SEM micrograph of a TiSiN coated tool after a 25 mm length of cut without laser assist, (b) Magnified SEM micrograph of (a), (c) EDS Spectra of TiSiN coated tool (from the region near the cutting edge) (feed rate: 220 mm/min, depth of cut: 25 mm, no laser).
and the wear of the cutting edge appears to be lower than most of the other coatings investigated in this study. The adhesion characteristics of the ZrN interlayer with the alumina coating needs to be further improved to reduce wear of this coating.
5. Conclusions This paper presented an experimental study of the wear behavior of coated tools in LAMM. Micro-milling experiments were carried out on A2 tool steel (62 HRc) using a wide range of commercially available (TiCN and TiAlN) and custom (Al2O3, Al2O3 þ ZrN) coated tools to evaluate their wear behavior. The following specific conclusions can be drawn from this study:
Commercially available coatings like TiCN perform poorly due Fig. 10. SEM micrograph of a new alumina (Al2O3) coated end mill (250 mm dia.).
been studied by researchers to promote adhesion between alumina and WC [18]. The micrograph in Fig. 12(a) shows that wear is reduced with the Al2O3 þZrN coating but some delamination of the coating still occurs near the cutting edge. The EDS spectra from the region indicated in Fig. 12(a) is shown in Fig. 12(b). The EDS spectra suggest that the ZrN layer is present even after 25 mm of cutting although the alumina coating has delaminated. Hence, the presence of the ZrN interlayer enhances tool life
to their inferior adhesion characteristics with the base material. Delamination is found to be the principal wear mechanism of TiCN, TiSiN, and Alumina (Al2O3) coated tools under the conditions investigated in this study. Wear of the tool coatings without laser assist is higher than with laser assist. This clearly indicates that coated micro-tools are able to withstand the higher temperatures and lower interfacial stresses experienced with laser assist. The principal wear mechanism of commercially available TiAlN coated tools is gradual abrasive wear with laser assist and delamination wear without laser assist. Alumina Al2O3 þZrN tools performed better than most of other coated tools in terms of flank wear, but there is still some delamination of the alumina layer. Hence, improvement in the
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Fig. 11. (a) and (b) SEM micrographs of Al2O3 coated tool after 25 mm of micro-milling, (c) EDS Spectra of Al2O3 coated tool (from region ‘A’ in Fig. 11(a)), (d) EDS Spectra of Al2O3 coated tool (from region ‘B’ in Fig. 11 (b)) after 25 mm length of cut with laser assist (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
Fig. 12. (a) SEM micrograph of alumina Al2O3 þZrN coated tool after 25 mm of micro-milling, (b) EDS spectra of alumina Al2O3 þ ZrN coated tool (from the region indicated in Fig. 12(a)) after 25 mm of length of cut with laser assist (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
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adhesion characteristics of the coating is needed to further enhance tool life in LAMM.
Acknowledgment The first two authors acknowledge support for this work from NSF Grant CMMI-0654369. The coated tools were provided by SECO Toolss and Entegris Specialty Coatings (Burlington, Massachusetts). References [1] D. Dornfeld, S. Min, Y. Takeuchi, Recent advances in mechanical micromachining, CIRP Annals—Manufacturing Technology 55 (2) (2007) 745–768. [2] T. Masuzawa, State of the art of micromachining, CIRP Annals—Manufacturing Technology 49/2 (2000) 473–488. [3] H. Weule, V. Huntrup, H. Tritschler, Micro-cutting of steel to meet new requirements in miniaturization, CIRP Annals—Manufacturing Technology 50/1 (2001) 61–64. [4] R. Singh, S.N. Melkote, Characterization of a hybrid laser-assisted mechanical micromachining (LAMM) process for a difficult-to-machine material, International Journal of Machine Tools and Manufacture 47 (2007) 1139–1150. [5] Y. Jeon, F.E. Pfefferkorn, Effect of laser preheating the workpiece on microend milling of metals, ASME Journal of Manufacturing Science and Engineering 130 (2008) 1–9, Paper no. 011004. [6] M. Kumar, S.N. Melkote, F. Hashimoto, G. Lahoti, Laser-assisted micro-milling of hard-to-machine materials, CIRP Annals—Manufacturing Technology 58 (2009) 45–48.
[7] A. Aramcharoen, P.T. Mativenga, S. Yang, K.E. Cooke, D.G. Teer, Evaluation and selection of hard coatings for micro-milling of hardened tool steel, International Journal of Machine Tools & Manufacture 48 (2008) 1578–1584. [8] P.J. Heaney, A.V. Sumant, C.D. Torres, P.W. Carpick, F.E. Pfefferkorn, Diamond coatings for micro-end mills: enabling the dry machining of aluminum at the micro-scale, Diamond & Related Materials 17 (2008) 223–233. [9] H.C. Carslaw, J.C. Jaeger, Conduction of Heat in Solids, 2nd ed., Oxford, Clarendon Press, 1959. [10] F. Cverna, Thermal Properties of Metals, ASM International, Materials Park, Ohio, 2002. [11] H.G. Woo, H.S. Cho, Three-dimensional temperature distribution in laser surface hardening process, Proceedings of the Institute of Mechanical Engineers 213 (1999) 695–712. [12] W.M. Steen, C.H.G. Courtney, Surface heat treatment of En8 steel using a 2 kW continuous-wave CO2 laser, Metals Technology 6 (12) (1979) 456–462. [13] D.L. Wissmiller, F.E. Pfefferkorn, Micro-end mill temperature measurement and prediction, Journal of Manufacturing Processes 11 (2009) 45–53. [14] K.D. Bouzakis, G. Skordaris, S. Gerardis, G. Katirtzoglou, S. Makrimallakis, M. Pappa, E. Lill, R. M’Saoubi, Ambient and elevated temperature properties of TiN, TiAlN and TiSiN PVD films and their impact on the cutting performance of coated carbide tools, Surface and Coatings Technology 204 (2009) 1061–1065. [15] H. Prengel, W.R. Pfouts, A.T. Santhanam, State of the art in hard coatings for carbide cutting tools, Surface and Coatings Technology 102 (1998) 183–190. [16] K. Maekawa, T. Obikawa, Y. Yamane, T.H.C. Childs, Metal Machining: Theory and Applications, John Wiley & Sons, New York, NY, 2000. [17] C.H.R.V. Kumar, P.K. Nair, B. Ramamoorthy, Characterization of multilayer PVD nano-coatings deposited on tungsten carbide cutting tools, International Journal of Advanced Manufacturing Technology 38 (2008) 622–629. [18] D. Jianxin, L. Jianhua, Z. Jinlong, S. Wenlong, Wear mechanisms of PVD ZrN coated tools in machining, International Journal of Refractory Metals & Hard Materials 26 (2008) 164–172.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Wear behavior of coated tools in laser assisted micro-milling of hardened steel Mukund Kumar a, Shreyes N. Melkote n,a, Rachid M’Saoubi b a b
George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, USA R&D Materials and Technology Development, SECO Tools AB, Fagersta, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 March 2012 Received in revised form 8 August 2012 Accepted 9 August 2012 Available online 30 August 2012
This paper presents a comparison of the performance of different cutting tool coatings for Laser Assisted Micro-Milling (LAMM). A thermal model is used to predict the temperature rise in the material removal surface which helps in analyzing the severity of the thermal conditions experienced by the cutting tool in LAMM. Machining experiments are then carried out to evaluate the wear behavior of different commercially available (TiCN, TiAlN) and customized coated tools (TiSiN, Al2O3, Al2O3 þ ZrN). These coatings were selected since they have the capability to withstand the temperatures experienced in LAMM. The results of micro-milling experiments indicate that commercially available coatings like TiCN perform poorly due to their inferior adhesion characteristics with the base material. Delamination is found to be the principal wear mechanism of TiCN, TiSiN, and Alumina (Al2O3) coated tools for the conditions investigated in this study. In addition, the results indicate that the wear performance of TiAlN and Al2O3 þ ZrN coated tools is superior. & 2012 Elsevier B.V. All rights reserved.
Keywords: Tool wear Coatings Laser assisted micro-milling
1. Introduction Micro-milling is increasingly being used to fabricate micromolds for injection molding [1,2]. Hardened steels and ceramics are the preferred materials for micro-molds because of their ability to withstand high cyclic thermal and mechanical loads [3]. In order to create features in these hard-to-machine materials, new hybrid processes have been proposed [4,5]. Laser Assisted Mechanical Micro-milling (LAMM) is one such process [6]. Recent work on LAMM has demonstrated its potential for micromachining hard materials [6]. However, a key limitation of the process today is the lack of affordable cutting tool materials that can withstand the high temperatures experienced in LAMM and also possess reasonable tool life. Conventional micro-milling tools (o500 mm dia.) are made of micro-grain tungsten carbide (WC–Co), whose strength starts to decrease at temperatures above 500 1C. In addition, tool wear becomes significant at high temperatures. A cost effective solution to this problem is the use of temperature-resistant coatings. The suitability of tool coatings employed in conventional macro- and micro-milling processes [7,8] for use in LAMM needs to evaluated since the heat generated in LAMM is significantly higher than in conventional processes. No work has been reported on the wear behavior of coated tools in LAMM.
n
Corresponding author. Fax: þ1 404 894 9342. E-mail address: [email protected] (S.N. Melkote).
0043-1648/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.wear.2012.08.011
Consequently, the wear performance of carbide micro-end mills with commercially available coatings (TiCN and TiAlN) and custom coatings (Al2O3, Al2O3 þZrN) is studied through micro-milling experiments carried out on hardened A2 tool steel (62 HRC). The worn tools are characterized using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) to understand the dominant wear mechanisms of the tool coatings.
2. Thermal modeling Prior to carrying out the tests, the temperature distribution in the workpiece surface directly in contact with the tool is analyzed via an experimentally calibrated moving heat source model to estimate the thermal loads experienced by the tool in LAMM and to aid in the selection of suitable cutting tool coatings. A standard moving heat source model is used to calculate the temperature distribution in the workpiece surface in contact with the tool (termed the material removal surface in this paper). Specifically, the temperature distribution is determined using an analytical model of a moving point heat source [9] over a semi-infinite medium as given by Eq. (1) Z Z 0 Tðx,y,zÞ ¼ a qððx0 ,y0 Þ=2pKsÞeðU=2kÞðsðxx ÞÞ dx0 dy0 , qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð1Þ s ¼ ðxx0 Þ2 þ ðyy0 Þ2 þ ðzÞ2 : where, a is the absorptivity, q is the heat intensity (W/m2), K is the thermal conductivity (W/m-K), k is the thermal diffusivity (m2/s), and
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U is the velocity of the heat source (m/s). The thermal conductivity of the work material is modeled as a temperature dependent function given by Eq. (2) [10]. K ¼ 18:7 þ 0:0138 T
ð2Þ
The temperature dependence of thermal conductivity requires an iterative method to solve Eq. (1). The laser beam is modeled as a 280 mm diameter Gaussian heat source and the laser power is fixed at 18 W. The distance between the trailing edge of the laser spot and the leading edge of the cutting tool is fixed at 60 mm. The temperature distribution below the workpiece surface calculated from the model assuming unit absorptivity is shown in Fig. 1. The scanned specimen is sectioned, polished and etched to reveal the microstructure shown in Fig. 2. It clearly shows the demarcation between the phase transformed or laser hardened region (white color) and the tempered region (black color) created by the laser scan. The depth of the phase transformed region, d, is measured using an optical microscope. The temperature at this depth is taken to be the Ac3 temperature of A2 tool steel, which is 793 1C. The absorptivity coefficient in the thermal model is calibrated by dividing the temperature at depth d computed from the thermal model by the nominal Ac3 temperature of A2 tool steel. This calibration approach has been used successfully by other researchers [11,12]. Using this approach, the absorptivity for the conditions given in Fig. 1 is found to be 0.793. Note that it is possible to predict the depth of the phase transformed region without measuring it each time, provided the temperature dependence of thermal conductivity and specific heat, and the dependence of surface roughness on the absorptivity of the laser beam are known precisely [10]. However, in practice it is often difficult to obtain these values for numerical computations. Hence, the above model calibration approach is used in this study [9]. Once the calibrated absorptivity is known, the thermal model can be used to determine the temperature variation in the curved
Fig. 2. Micrograph of laser scanned surface showing the phase transformed region (white color) for the conditions given in Fig. 1 (d is the depth of phase transformed region).
Fig. 3. (a) Position of tool with respect to the laser beam (A–B indicates the line along which the temperature is predicted), (b) Predicted temperature variation in the workpiece along A–B and E–F (laser power: 18 W, spot size: 280 mm, scan speed: 100 mm/min, laser-tool distance: 100 mm).
Fig. 1. Temperature distribution (in 1C) in the X–Z plane (shown in Fig. 3) due to a laser scan along the indicated direction (laser power: 18 W, spot size: 280 mm, scan speed: 100 mm/min, laser-tool distance: 100 mm, absorptivity: 1).
material removal surface at the leading edge of the tool. The relative positions of the laser beam and the tool used in the LAMM experiments are shown in Fig. 3(a). Under these conditions, the temperature distribution is determined at the front edge of the tool along the lines A–B and E–F in Fig. 3(a). The temperature rise is found to vary between 300 and 450 1C on the curved material removal surface, as seen in Fig. 3(b). However, this temperature rise is only due to laser heating. The heat
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Table 1 Experimental conditions. Cutting tool Spindle speed Feed rate Depth of cut Workpiece material Laser power and spot size Distance between the center of the laser spot and the leading edge of the tool
200 mm dia. WC–Co, 2 flute end mills, and 250 mm dia. WC–Co 2 and 4 flute end mills 50,000 rpm 200 mm dia. tools—220 mm/min 250 mm dia. tools—100 mm/min 25 mm A2 Tool Steel (62 HRc) 18 W, 280 mm 200 mm
generated due to cutting is not considered in this study. Thus, the temperature rise experienced by the tool will be higher than shown in Fig. 3. It is evident from Fig. 3(b) that the temperature rise of 300–450 1C in LAMM is significant when compared to the estimated temperature rise of only 100–200 1C in micro-milling of 1018 steel and Al6061-T6 without laser heating [13]. Consequently, the coated tools must be capable of withstanding such high temperatures in the LAMM process for them to be viable.
Table 2 Cutting tool coatings. Commercially available Custom coated
TiAlN, TiCN TiSiN, Al2O3, Al2O3 þ ZrN
3. Micro-milling experiments with coated tools Laser assisted micro-milling experiments were conducted on A2 tool steel (62 HRc) using the experimental conditions given in Table 1. The tools used were 200 and 250 mm diameter end mills obtained from different tool manufacturers. The slotting experiments were conducted at manufacturer recommended feed rates and depths of cut. A few tests were conducted without laser assist to compare with the laser assist cases. All experiments were performed dry. The tool coatings evaluated in this study are listed in Table 2. These coatings were selected since they have the capability to withstand the high temperatures experienced by the tool in the LAMM process. Note that TiAlN and TiCN coatings are available from select cutting tool manufacturers. Tools were also custom coated with TiSiN, amorphous alumina (Al2O3), and Al2O3 þZrN in collaboration with a cutting tool manufacturer and a coatings supplier, respectively. All coatings were deposited on the cutting tools using the Physical Vapor Deposition (PVD) process. TiAlN and Al2O3 were PVD sputter coated, and TiSiN was coated using the arc evaporation technique. The TiSiN coating has been recently shown to outperform TiAlN and TiN coatings in terms of better abrasive wear resistance and fatigue behavior in conventional milling experiments [14]. The Al2O3 coating was selected because of its higher thermal stability and chemical inertness [15]. The thickness of TiSiN and Al2O3 coatings ranged from 2.5 to 3 mm. The thickness of the ZrN interlayer, which is used to promote adhesion of the alumina coating to the carbide substrate, is around 0.5 mm. The worn tools were characterized by Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) to understand the dominant wear mechanisms.
4. Wear behavior of coatings 4.1. Uncoated tool Fig. 4(a) shows an SEM micrograph of the uncoated tungsten carbide micro-end mill after a 25 mm length of cut. The micrograph reveals extensive rubbing on the flank face of the uncoated tool (indicated by the arrows). Edge rounding without edge chipping is also observed, which indicates gradual abrasive wear of the tool. In addition, as seen in Fig. 4(a), adhesion of the softened work material to the cutting tool surfaces is also observed.
Fig. 4. (a) SEM micrograph of uncoated tool after 25 mm of micro-milling, (b) EDS spectra of the uncoated tool (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm) from the region indicated in Fig. 4(a). The EDS spectra obtained from this region confirms the presence of oxygen (O).
Visual examination of the used tool indicates some discoloration, which is attributed to oxidation of the uncoated carbide tool. Tungsten carbide begins to oxidize at temperatures above 500 1C [16]. As predicted by the thermal model, the workpiece is heated
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Fig. 5. (a) SEM micrograph of a new TiCN coated tool, (b) SEM micrograph of a TiCN coated tool after a 25 mm length of cut, (c) EDS Spectra of TiCN coated tool (from the top left region near the cutting edge indicated in Fig. 5(b)), (d) EDS Spectra of TiCN coated tool (from the flank face as indicated in Fig. 5(b)) (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
Fig. 6. (a) SEM micrograph of the TiAlN (SECOs Mega-T) coated tool after a 25 mm length of cut with laser assist, (b) EDS Spectra of TiAlN coated tool (from around the cutting edge), (c) End cutting edges indicating minimal wear (feed rate: 220 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
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to a temperature of 450 1C. In addition, there is temperature rise due to shear and frictional heating produced by the cutting process. Hence, the total temperature rise is expected to be closer to 500–600 1C in LAMM. The ability of the tool to resist wear at these high temperatures will directly determine tool life.
Fig. 5(d) indicates the overwhelming presence of titanium (Ti), carbon (C) and nitrogen (N), which confirm the composition of the coating material. Based on these findings, delamination wear is the dominant mechanism that governs the wear behavior of TiCN coated micro-tools in LAMM.
4.2. TiCN coated tool
4.3. TiAlN coated tools
SEM micrographs of the TiCN coated tool before and after a 25 mm length of cut with laser assist are shown in Fig. 5(a) and (b), respectively. Fig. 5(b) shows clear evidence of coating delamination near the cutting tool edge. The coating has been completely removed but the cutting edge is still intact. In addition, there is rounding of the cutting edges indicating gradual tool wear. Improvement in wear performance will be obtained if the surface roughness and adhesion of the coating to the WC–Co substrate can be enhanced. Delamination of the coating is confirmed by the EDS spectra (Fig. 5(c) and (d)) corresponding to the two regions marked in Fig. 5(b). The spectra shown in Fig. 5(c) reveals the presence of only tungsten (W) and carbon (C), which indicate that the TiCN coating has been removed from these regions. These observations are in line with the results of Aramcharoen et al. [7], who observed that the TiCN coating delaminates because of poor adhesion to the base substrate when micromilling hardened tool steel (45 HRc) with a 500 mm micro-milling tool, albeit without laser assist. This is also confirmed by their pin-on-disc experimental results [7], which show that the TiCN coating has a higher hardness (3178 HV) and a lower adhesive failure load of 48 N compared to the TiN coating, which has a hardness of 2738 HV and a adhesive failure load of 60 N.
A SEM micrograph of the TiAlN coated tool (SECOs Mega-T) after 25.4 mm of cutting with laser assist is shown in Fig. 6(a). The principal wear mechanism of this coating is gradual abrasive wear, which is confirmed by the edge rounding observed in the SEM image. The EDS spectra from the cutting edge (shown in Fig. 6(b)) shows the presence of TiAlN. No delamination of the coating is observed, which indicates good adhesion of the coating to the substrate. In addition, significant adhesion of the softened work material to the cutting tool surfaces is observed and is confirmed by the presence of Iron (Fe) and (O) peaks in the EDS spectra. Fig. 7(a–c) show SEM micrographs of the TiAlN coated tool after a 25.4 mm length of cut without laser assist. It is evident that the coating is unable to withstand the tool stresses and therefore delaminates, which suggests that tool wear is higher without laser assist. At lower cutting temperatures (without laser assist), aluminum oxide is unlikely to form on the TiAlN coating, resulting in increased tool wear [7]. In contrast, as seen in Fig. 6(a), the coating is able to withstand the higher temperatures and lower interfacial stresses experienced by the cutting tool in LAMM. This might be due to the formation of an Al2O3 film on the TiAlN coating, which yields enhanced wear performance.
Fig. 7. SEM micrograph of the TiAlN (Mega-T) coated tool after a 25 mm length of cut without laser assist. (a) Delamination of coating, (b) Magnified SEM micrograph of (a), (c) Chipping of cutting edge, (d) EDS Spectra of TiAlN coated tool (from around the cutting edge) (feed rate: 220 mm/min, depth of cut: 25 mm).
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Fig. 8. (a) SEM micrograph of a TiSiN coated tool after a 25 mm length of cut with laser assist, (b) Magnified SEM micrograph of (a), (c) EDS Spectra of TiSiN coated tool (from the region near the cutting edge) (feed rate: 220 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
4.4. TiSiN coated tool Fig. 8(a) and (b) show SEM micrographs of the TiSiN coated tool after 25.4 mm of machining with laser assist. The coating is found to delaminate under the influence of the machining stresses. Additionally, the EDS spectra obtained from the region near the cutting edge is shown in Fig. 8(c) and confirms that the coating has peeled off. Therefore, delamination is the principal wear mechanism of this coating. Adhesion of the TiSiN coating with the WC–Co substrate has to be further improved for the coating to perform better. This has also been observed in conventional milling where the TiSiN coatings exhibited poor adhesion [14]. Fig. 9(a) and (b) show SEM micrographs of the TiSiN coated tool after 25.4 mm of machining without laser assist. The coating near the cutting edge has peeled off, which is confirmed by the EDS spectra shown in Fig. 9(c). Extensive adhesion of the work material with the cutting tool is observed, which can also cause delamination of the coating. Based on these results, it can be concluded that delamination is the dominant wear mechanism of the TiSiN coating in micro-milling with and without laser assist.
this study. Fig. 11(a) and (b) show SEM micrographs of the worn tool after micro-milling a 25 mm long slot with laser assist. The micrographs clearly indicate peeling of the Al2O3 coating. This is thought to be due to a combination of excessive rubbing at the micro-scale combined with poor adhesion of the coating to the tungsten carbide substrate. Kumar et al. [17] have used indentation to study the adhesion of Al2O3 coatings to a carbide substrate and report that the adhesion strength of a single layer Al2O3 coating to the carbide substrate is HF6 on a scale of HF1 to HF6, where 1 indicate the strongest adhesion. The EDS spectra shown in Fig. 11(b) from the region marked ‘A’ in Fig. 11(a) clearly indicates complete loss of the alumina coating. The EDS spectra from region ‘B’, shown in Fig. 11(d), shows the presence of constitutive elements (Al and O) of the coating. Note that the EDS spectra are obtained from a maximum substrate depth of 2 mm. The absence of tungsten and carbon in Fig. 11(d) confirms that the remaining thickness of the coating in this region is more than 2 mm. The cutting edge is still intact after 25 mm of cutting under the machining conditions listed in Table 1. Delamination due to inadequate adhesion strength of the coating appears to be the principal wear mechanism of the alumina coated tools in LAMM.
4.5. Alumina (Al2O3) coated tools 4.6. Alumina (Al2O3) þZrN coated tool Alumina (Al2O3) coating of thickness 2.5–3 mm was deposited on a two flute WC–Co micro-end mill using the PVD process. The coated tool before machining is shown in Fig. 10. The coating appears to be relatively smooth compared to the other coatings in
To improve adhesion of the alumina coating to the carbide substrate, a thin layer of ZrN (600 nm) was deposited on the tool before depositing the alumina layer. Note that ZrN coating has
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Fig. 9. (a) SEM micrograph of a TiSiN coated tool after a 25 mm length of cut without laser assist, (b) Magnified SEM micrograph of (a), (c) EDS Spectra of TiSiN coated tool (from the region near the cutting edge) (feed rate: 220 mm/min, depth of cut: 25 mm, no laser).
and the wear of the cutting edge appears to be lower than most of the other coatings investigated in this study. The adhesion characteristics of the ZrN interlayer with the alumina coating needs to be further improved to reduce wear of this coating.
5. Conclusions This paper presented an experimental study of the wear behavior of coated tools in LAMM. Micro-milling experiments were carried out on A2 tool steel (62 HRc) using a wide range of commercially available (TiCN and TiAlN) and custom (Al2O3, Al2O3 þ ZrN) coated tools to evaluate their wear behavior. The following specific conclusions can be drawn from this study:
Commercially available coatings like TiCN perform poorly due Fig. 10. SEM micrograph of a new alumina (Al2O3) coated end mill (250 mm dia.).
been studied by researchers to promote adhesion between alumina and WC [18]. The micrograph in Fig. 12(a) shows that wear is reduced with the Al2O3 þZrN coating but some delamination of the coating still occurs near the cutting edge. The EDS spectra from the region indicated in Fig. 12(a) is shown in Fig. 12(b). The EDS spectra suggest that the ZrN layer is present even after 25 mm of cutting although the alumina coating has delaminated. Hence, the presence of the ZrN interlayer enhances tool life
to their inferior adhesion characteristics with the base material. Delamination is found to be the principal wear mechanism of TiCN, TiSiN, and Alumina (Al2O3) coated tools under the conditions investigated in this study. Wear of the tool coatings without laser assist is higher than with laser assist. This clearly indicates that coated micro-tools are able to withstand the higher temperatures and lower interfacial stresses experienced with laser assist. The principal wear mechanism of commercially available TiAlN coated tools is gradual abrasive wear with laser assist and delamination wear without laser assist. Alumina Al2O3 þZrN tools performed better than most of other coated tools in terms of flank wear, but there is still some delamination of the alumina layer. Hence, improvement in the
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Fig. 11. (a) and (b) SEM micrographs of Al2O3 coated tool after 25 mm of micro-milling, (c) EDS Spectra of Al2O3 coated tool (from region ‘A’ in Fig. 11(a)), (d) EDS Spectra of Al2O3 coated tool (from region ‘B’ in Fig. 11 (b)) after 25 mm length of cut with laser assist (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
Fig. 12. (a) SEM micrograph of alumina Al2O3 þZrN coated tool after 25 mm of micro-milling, (b) EDS spectra of alumina Al2O3 þ ZrN coated tool (from the region indicated in Fig. 12(a)) after 25 mm of length of cut with laser assist (feed rate: 100 mm/min, depth of cut: 25 mm, laser power: 18 W, spot size: 280 mm).
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adhesion characteristics of the coating is needed to further enhance tool life in LAMM.
Acknowledgment The first two authors acknowledge support for this work from NSF Grant CMMI-0654369. The coated tools were provided by SECO Toolss and Entegris Specialty Coatings (Burlington, Massachusetts). References [1] D. Dornfeld, S. Min, Y. Takeuchi, Recent advances in mechanical micromachining, CIRP Annals—Manufacturing Technology 55 (2) (2007) 745–768. [2] T. Masuzawa, State of the art of micromachining, CIRP Annals—Manufacturing Technology 49/2 (2000) 473–488. [3] H. Weule, V. Huntrup, H. Tritschler, Micro-cutting of steel to meet new requirements in miniaturization, CIRP Annals—Manufacturing Technology 50/1 (2001) 61–64. [4] R. Singh, S.N. Melkote, Characterization of a hybrid laser-assisted mechanical micromachining (LAMM) process for a difficult-to-machine material, International Journal of Machine Tools and Manufacture 47 (2007) 1139–1150. [5] Y. Jeon, F.E. Pfefferkorn, Effect of laser preheating the workpiece on microend milling of metals, ASME Journal of Manufacturing Science and Engineering 130 (2008) 1–9, Paper no. 011004. [6] M. Kumar, S.N. Melkote, F. Hashimoto, G. Lahoti, Laser-assisted micro-milling of hard-to-machine materials, CIRP Annals—Manufacturing Technology 58 (2009) 45–48.
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